The proposed research plan focuses on neutral amino acid transport in vascular endothelial cells; transport mechanisms and modes of regulation. This proposal stems from our observation that at least six different routes for neutral amino acid transport are present in cultured endothelial cells from bovine aorta. In cells which have been growth arrested by serum removal, the activity of these transport pathways are markedly enhanced by serum addition. this suggests that neutral amino acids have a role in endothelial cell growth and proliferation. However, preliminary data on four amino acids which are usually typical substrates of these transport systems (i.e. alanine for system ASC, MeAIB for system A, glutamine for system N and leucine for system L), indicate that substantial overlapping exists in substrate transport among these carrier systems. Therefore, selective model substrates need to be identified so that a detailed characterization of transport mechanisms can be carried out. The absence of model substrates precludes the elucidation of possible mechanisms of regulation of amino acid transport by serum. Specific aims include: 1) characterization of the cellular mechanisms involved in neutral amino acid transport in vascular endothelial cells derived from the bovine aorta, using selective model substrates derived from kinetic studies and competition experiments; 2) evaluation of the stimulatory effect of serum on amino acid transport; 3) investigation of the temporal relation between Na+/H+ exchange activation, pHi-changes and amino acid uptake enhancement by serum; 4) evaluation of possible second messenger systems involved in the regulation of amino acid transport by serum and 5) to offer graduate students the opportunity to be exposed to a line of research that involves membrane transport and its biochemical, physiological and pharmacological aspects. These studies should provide new information on mechanisms for neutral amino acid transport in vascular endothelial cells and their possible mode(s) of regulation. In addition, these studies may provide new insights into the role of amino acid transport in the control of endothelial cell growth and proliferation. The latter appears to be critical to have a better understanding of angiogenesis.